Patent GB-847739-A (GB)
Title: Improvements in or relating to cathode ray storage tubes
IPC: H01J31/60
Filed: 1958-07-31
Abstract: 847,739. Cathode ray storage tubes. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. July 31, 1958 [Aug. 8, 1957], No. 24669/58. Class 39(1). A cathode ray storage tube circuit of the low velocity orthicon type for operation successively in writing reading and erasing comprises a target consisting of a metal back plate 14, Fig. 1, having an insulating layer 13 thereon and a fine metal grid 12 on the insulating layer input signals are applied to the gun control electrode or to the target back plate and output is taken from the target grid, the cathode being at earth potential for reading and writing (slow velocity scanning, secondary emission ratio <1) and at negative potential relative to earth (high velocity beam, secondary emission ratio >1) for erasing and means for bringing the target back plate to a positive potential higher than the potential of the target grid for writing when the input signals are applied to the control electrode and to earth potential either for reading and erasing operations when the input signals are applied to the gun control electrode or for all operations when the input signals are applied to the target back plate. An electrostatic 'lens' system 8, 9, 10 is provided for bending the path of the electron beam so that the electrons strike the target perpendicularly i.e. orthicon operation. There is a single writing operation followed by one or several non- destructive readings and one or several erasings with or without time intervals between these operations. Several signals may be read consecutively with or without time intervals between the readings. For writing grid 12 is held at 5 volts by DC source 22 switch 23 is in the dotted line position to apply + 20 volts to the target back plate 14 and the cathode is earthed by switch 19. The Wehnelt electrode 4 is biased to cut off (-70 volts with respect to cathode) and the beam is turned on by the input signal. The slow electrons are collected by grid 12 (at + 5 volts) while the faster electrons charge up 13 by secondary emission the secondary emission ratio being less than unity so a negative charge image is bult up the maximum amplitude of the signal bringing the potential of the insulating layer 13 to zero volts. After writing switch 23 is moved over to earth back plate 14 so the potentials all drop by 20 volts (maximum signal amplitude points drop to -20 volts, unmodulated picture points to -15 volts) and a constant intensity slow electron beam scans. The electrons from the zero potential (unmodulated) picture points are all collected by grid 12, while the negative potential modulated picture points repel all the elctrons approaching them and the repelled electrons are divided into two parts one part being collected by grid 12 and the other part returning through grid 11 and orthogonalising electrodes 10, 9, 8. The ratio of division of the rejected electrons depends on the potential of the corresponding picture point. Since no electrons strike the insulating surface of 13 reading does not destroy the charge image on 13. The output across 21 is therefore a maximum for picture points not signal modulated and decreases for the various values of stored charges on the other picture points to a zero -value for maximum signal . The half tones can therefore be reproduced by establishing a uniform relationship between each intermediate value of signal amplitude and current in resistor 21. For erasing the cathode is put at -400 volts by charging over switch 19, switch 23 being kept earthed The gun and collimating electrode potentials are all decreased by equal amounts, while the potential of the Wehnelt electrode 4 is increased by a constant amount. An electron beam of fast electrons thus scans the target to erase the negative charge image and bring the potentials of all picture points to approximately the potential of grid 12 which has remained at +5 volts throughout the reading writing and erasing operation. The beam is scanned, in all phases of operation, by two pairs of deflecting coils 25. In a modification the input is applied to the backing plate 14 as shown at 29<SP>1</SP>, 30<SP>1</SP>. The reading and erasing regimes are unaltered, but for writing the potential of the Wehnelt electrode 4 must be increased to allow the electron beam to pass, the target potential being stabilised at cathode potential.